The ability to controllably tune the work function of transparent conductors is essential for optimizing the efficiency of thin film solar photovoltaics. Here we use Kelvin force microscopy on lithographically patterned ZnO:Al thin films to measure the dependence of the work function on oxygen content during synthesis. Films were synthesized by low temperature reactive sputtering from a Zn:Al 1.2 wt% target. At optimal oxygen content the resistivity is 3 x 10(-4) Omega cm with nearly 90% optical transmission in the 400-1100 nm wavelength range relevant to solar photovoltaics. We find that the expected relationship between band filling and work function breaks down in the vicinity of optimal oxygen stoichiometry. For oxygen-rich ZnO:Al films we measure large work functions close to 5 eV. Our results suggest a method for fabricating transparent oxide electron conductors with large, tunable work functions that could be of relevance in designing electrodes for solid state energy conversion technologies. (C) 2010 Elsevier B.V. All rights reserved.